1. Field of the Invention
The present invention relates to a container lid structure, and more particularly to a container lid structure provided with a lid opening tab. More specifically, the present invention pertains to a container lid structure which is totally or mostly made of plastic material.
2. Description of the Prior Art
A container lid structure having a finger gripping tab for opening the container has been widely adopted for example in a metal can. It has also been proposed for example by the Japanese patent laid-open application No. 57-163641, the Japanese patent publication No. 58-52897 and the Japanese patent laid-open application No. 59-31131 to provide a container lid structure having such a container opening tab with a plastic forming technique. The proposed container lid structure includes a lid frame adapted to be attached to a container barrel and a lid plate encircled by the lid frame, a score line being formed along a border between the lid plate and the lid frame to facilitate breakage in opening the lid. The score line is in the form of a groove formed on the upper surface of the lid structure.
The score line thus formed on the upper surface of the lid is of a groove configuration having a low strength so that it can cause a deformation in the lid frame in storage or transportation of the container when the container is subjected to a shock load by being dropped or hit to another article. In a worst case, the lid may be broken at the score line causing the content of the container to be discharged. It should be noted that such problems are apt to occur when the lid structure is made of a plastic material because the plastic material has a lower strength.
Since the score line is of a groove configuration exposed to the upper surface of the lid, it is likely that dust is accumulated in the groove during storage of the container providing an insanitary appearance and decreasing the commercial value. It should further be noted that due to the low strength there is a further problem in that the so-called tamper resist property, that is, the property which can prevent others from piercing an injection needle through the score line, may be lowered.
Another problem experienced in a conventional plastic lid structure is that the opening tab is formed by a process which is separate from the process for the manufacture of the lid plate and attached to the lid plate afterwards. In the conventional lid structure, the manufacturing process therefore includes an increased number of steps and difficulties have been experienced in trying to decrease the manufacturing cost.